11.9 - Sex-Linked Inheritance

What a sex-linked characteristic is

• A sex-linked characteristic is a feature whose gene is on a sex chromosome, not on an ordinary (autosomal) chromosome.
• In practice this almost always means the X chromosome.
• Because the gene sits on a sex chromosome, the characteristic turns up more often in one sex than the other.

Why the two sex chromosomes matter

• The Y chromosome is much smaller than the X chromosome and carries far fewer genes.
• Most genes on the sex chromosomes are found only on the X, with no matching version on the Y.
• Males (XY) - Have one X and one Y, so for a gene found only on the X they have just a single allele. That allele is expressed even if it is recessive, because there is no second X carrying a dominant allele to mask it.
• Females (XX) - Have two X chromosomes, so they have two alleles for that gene. A recessive allele on one X can be masked by a dominant allele on the other.

Why males are affected more often

• A male needs only one copy of the faulty recessive allele to be affected, because his single X has no partner to mask it.
• A female needs two copies, one on each X, to be affected.
• A female who carries a single copy of the recessive allele is not affected herself, because the dominant allele on her other X masks it - but she can still pass the faulty allele on to her children.
• Such a female is called a carrier.

Following a sex-linked cross

• Haemophilia is a sex-linked condition - a blood-clotting disorder caused by a recessive allele carried on the X chromosome.
• Because the allele sits on a sex chromosome, it's written as a superscript on the X:
  • X^H - The normal allele
  • X^h - The haemophilia allele
  • Y - Carries no allele for this gene, so it's written as a plain Y
• A sex-linkage cross must always use symbols showing both the sex chromosome and the allele on it (e.g. X^h Y).
• A male has only one X, so a male carrying the haemophilia allele (X^h Y) has the condition - which is why most people with haemophilia are male.

Cross 1 - Affected father × unaffected non-carrier mother

Affected (haemophiliac) father X^h Y × unaffected non-carrier mother X^H X^H.

• Every daughter is X^H X^h - an unaffected carrier.
• Every son is X^H Y - unaffected.
• None of the children has haemophilia, but every daughter carries the allele - this is how an affected father passes it on to grandchildren through his daughters.

Cross 2 - Unaffected father × carrier mother

Unaffected father X^H Y × carrier mother X^H X^h.

• Four equally likely offspring, giving a ratio of 3 unaffected : 1 affected.
• Only a son is affected: a 1 in 4 (25%) chance for any child, rising to 1 in 2 (50%) once you know the child is a boy.
• No daughter has haemophilia, because each daughter inherits the father's normal X^H.

Why haemophilia is usually found only in males

• A male needs just one faulty X to be affected.
• A female needs a faulty allele on both X chromosomes, so she must inherit one from each parent - which happens only very occasionally.